1. Field of the Invention
The present invention relates to cells, and more particularly, relates to a cell formed by using a laminate film as an exterior packaging material.
2. Description of the Related Art
In recent years, a great number of portable electronic apparatuses such as camera-incorporated VTRs (videotape recorders), mobile phones, and laptop computers have been introduced into the market, and trends toward miniaturization of the portable electronic apparatuses and reduction in weight thereof have been positively pursued. Concomitant with the trends described above, cells used as power sources of portable electronic apparatuses are rapidly increasing in demand, and in order to realize miniaturization of the apparatuses and reduction in weight thereof, cell engineering has been requested to reduce the weight and thickness of a cell and to enable it to be efficiently accommodated in a limited space of a portable electronic apparatus. As a cell capable of satisfying the requests described above, for example, lithium ion cells, which have a high energy density and output density, may be mentioned as one of the most preferable cells.
Among the various cells, a cell having a high degree of freedom of shape, a thin sheet-type cell having a large area, a thin card-type cell having a small area, and the like have been desired; however, as long as a related method is employed which uses a metal can as an exterior packaging material, it has been difficult to manufacture thin cells.
In order to overcome the problem described above, for example, a cell containing a material imparting adhesive properties to an electrolyte, and a cell containing a polymer gel electrolyte have been investigated. In the cells described above, the electrode and the electrolyte are tightly adhered to each other, and hence the contact state therebetween can be ensured. Accordingly, a thin cell can be manufactured by using an exterior packaging film such as an aluminum laminate film.
Heretofore, as applications of laminate films, for example, food, pharmaceuticals, and films may be mentioned; however, due to expiration dates given to products mentioned above, long-term reliability may not be always required for the laminate films in many cases. On the other hand, since secondary cells are used for a long time while charge and discharge are repeatedly performed, the lamination film is particularly requested to have good long-term reliability.
FIG. 1 is a schematic perspective view showing a cell which uses a laminate film as an exterior packaging material. Reference numeral 1 indicates a cell packaged in the laminate film. Since the laminate film has no electrical conductivity, electrode terminals 2 are necessarily provided along a side at which two parts of the laminate film are overlapped with each other so as to extend outside. In the state as described above, two parts of an interior resin film of the laminate film are disposed to face each other, and thermal welding is performed along a peripheral portion of a cell element, so that air-tight sealing can be performed. In this case, when the width of the film which is sealed by the thermal welding is decreased, a larger cell element can be designed, and hence, a cell having a higher capacity can be obtained.
FIG. 2 shows one example of a main structure of a laminate film 10. A metal foil indicated by reference numeral 11 is provided between a resin film 12 and a resin film 13 to form a multilayer film having humidity resistance and insulating properties. As the resin film 12 provided outside, nylon or polyethylene terephthalate) is used due to its fine appearance, toughness, and flexibility. The metal foil 11 has an important role of protecting a content packaged in the laminate film from entering moisture, oxygen, and light, and in consideration of light-weight properties, elongation properties, price, and processability, aluminum (Al) has been most widely used. Two parts of the interior resin film 13, facing each other, are melted by application of heat or ultrasonic sound so as to be welded to each other; hence a polyolefin resin is preferably used, and a cast polypropylene (CPP) resin has been used in many cases. Adhesive layers 14 may be provided between the metal foil 11 and the resin films 12 and 13, whenever necessary.
When a cell element is packaged in the laminate film 10, followed by thermal welding, the inside CPP layer is melted, and adhesion is carried out. However, a metal of the electrode terminal 2 extending from the cell element has poor adhesion properties to the CPP. Hence, as shown in FIGS. 1 and 3, in order to improve the adhesion to the CPP, resin material is adhered to two surfaces of the electrode terminal 2. This resin material is called a sealant 3.
As problems which may arise when an aluminum laminate film is used as an exterior packaging material, for example, since moisture is liable to infiltrate the cell to cause unfavorable electrolytic chemical reaction therein, degradation in cell properties may be mentioned. Moisture does not infiltrate through an Al layer and mostly infiltrates the interior resin (CPP) portion.
The amount of infiltrating moisture is proportional to a cross-sectional area (cross-sectional area of the CPP layer) of an infiltration path and is reciprocally proportional to the length thereof (sealing width). Accordingly, in order to prevent moisture infiltration, it is necessary that the thickness of the CPP layer be decreased to decrease the cross-sectional area or that the sealing width, that is, the path length, be increased. In view of increase in cell capacity, in order to reduce the amount of infiltrating moisture, it is more preferable that the infiltration path be narrowed by decreasing the thickness of the CPP layer. In this case, since the thickness of the laminate film itself is decreased, the size of an entire cell can be designed larger, and as a result, the capacity thereof can be increased.
However, when the sealing width is decreased, it becomes disadvantageously difficult to ensure the strength of a sealed portion. For solving this problem, it may be required to perform thermal welding using a metal block heater having a large heat capacity. When a related laminate film having a general thickness is used, since the interior resin film 13 has a thickness to a certain extent, even when the electrode terminal 2 is clamped by the metal block heater in thermal welding, the resin film 13 may absorb the pressure given thereby. However, when the interior resin film 13 has a small thickness, a large pressure is given particularly to a part of the electrode terminal 2 clamped by the metal block heater, and as a result, the electrode terminal 2 may be sheared or the electrode terminal 2 may penetrate the resin film 13 to cause short-circuiting with the metal foil 11.
Hence, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-348695, when recesses 21 are formed in a metal block heater 20 at positions corresponding to the electrode terminals 2, and thermal welding is performed while the recesses 21 and the electrode terminals 2 are being properly positioned, the problem as described above may be solved (see FIGS. 4A and 4B).